The X-ray image (CR image) for medical treatment is usually so shot that the portions where a transmission amount of X-ray is large appear dark, and is read by medical doctors. In the thus shot X-ray image, however, highly bright portions appear to be dazzling. Therefore, doctors who have to read many X-ray images a day often cover the highly bright portions with a paper for the purpose of lowering fatigue in the eyes. However, the highly bright portions have low detection sensitivity of the X-rays from the first. Besides, human eyes have low sensitivity to the highly bright portions. Therefore, it is difficult for doctors to read the highly bright portions.
In recent years, digitization of images for medical use has been developed, and the following image processing has been performed to solve the above-mentioned problem. That is, to read dark portions in detail, the image is so processed as to widen a range of low brightness by sacrificing the highly bright portions. To read bright portions in detail, on the other hand, the image is so processed as to widen a range of high brightness to cover even low brightness by sacrificing the lowly bright portions. With this image processing, however, either the dark portions or the bright portions become clear, but it is impossible to simultaneously read the dark portions and the bright portions, leading a possibility of overlooking the lesion sites.
In order to make the whole image more easy to read, therefore, there have been proposed methods called dynamic range filter for suppressing the highly bright portions and unsharp masking for enhancing the contrast (see Lecture 14 on the Science of Medical Radiogenics, “Medical Image Engineering”).
The dynamic range filter is not capable of enhancing the contrast of fine texture in the highly bright portions. Therefore, the contrast becomes weak in the bright portions, and the image as a whole becomes to appear flat.
The unsharp masking, on the other hand, creates an averaged blurred image or creates a blurred image utilizing the frequency conversion, and enhances a difference between the original image and the blurred image to improve the contrast. The highly bright portions, however, are still dazzling. In the portions where the contrast is high from the first, there occurs Gibbs' phenomenon as shown in FIG. 18, leading a possibility of misdiagnosis. In the case of a CR image of the chest shot in a health examination, the contrast is very strong along the circumference of the lung. If the unsharp masking is effected, therefore, a shadow-like portion appears along the edge of the lung, leading a possibility to misdiagnose as pneumothorax. By effecting the unsharp masking, further, an upper limit value of brightness is often exceeded in the highly bright portions.
In view of the above-mentioned problems, therefore, it is an object of the present invention to provide an image processing apparatus and an image processing program capable of adapting the contrast and brightness of the whole image to the sensitivity of human eyes, while preventing Gibbs' phenomenon in the high contrast portions.